Generally, we tend to think of migration as an event exclusively linked to complex organisms (like mammals or birds). But there are always exceptions: the North American populations of the monarch butterfly (Danaus plexippus) cover a distance of almost 5000km (more than some complex animals!) in order to reach their hibernation areas, where there can be concentrated thousands of specimens during the winter. Unfortunately, the migration phenomenon depend on many factors that are being damaged by anthropogenic pressure nowadays, so that the future of these populations and also their migration are in danger.
Throughout this article, you will learn some of the most curious biology traits of these organisms, the main causes that could be endangering their populations and the consequences that this would entail.
The monarch butterfly (Danaus plexippus) is a butterfly of the Nymphalidae family. It’s also probably one the most well-known butterflies of North America due to its long migration, that their specimens perform from the north of EEUU and Canada to California coast and Mexico, covering a distance of almost 5000km to reach their hibernation areas. It’s, by far, the insect that performs the widest and large migration of all.
Although the North American populations of this species are the most known worldwide due to their migration pattern, there are also monarch butterflies in some Atlantic islands (Canary islands, Azores and Madeira), and sometimes also as eventual migrators that reach the coasts of Western Europe (United Kingdom and Spain). Moreover, they were introduced in New Zealand and Australia during the XIX century.
The life cycle of this species is very unique. First of all, they’re considered specialist butterflies: they lay their eggs exclusively over plants of the Asclepias genus (also known as milkweeds), and their newborn caterpillars (which are black, white and yellow striped) feed only on these plants. This is a very interesting fact, because the plants of this genus contain cardiac glycosides that are progressively assimilated by the caterpillar tissues, which let them to acquire a disgusting taste that prevents them to be predated. This taste will last during their adult phase.
Once completed the larva phase, the metamorphosis take place so that the caterpillars become adult butterflies colored in black, white and orange. Both caterpillar and butterfly color patterns carry out a communicative function: it’s a mechanism to warn other animal of their toxicity, fact which is known as aposematic mimicry (this phenomenon is very frequent in a lot of group of animals, even in mammals).
The adult phase also has some particularities: during the mating season (April-August) some generations of adults are generated, and each of them has a life expectancy of a few weeks, more or less. Then, an awesome event takes place: the butterflies of the generation born at the end of August (when temperatures get low and days became shorter) stops the maturing process of their reproductive organs (phenomenon known as diapause) so they can spend their energy on enlarging their life expectancy to 9 months. This generation is known as “Methuselah generation” due to its longevity.
The increase of their longevity allows this generation to cover the long distance required to reach the hibernation areas during the autumn (Mexico and California coast) and then to come back to the north of America at the end of the winter.
A ROUND TRIP: THE GREAT MIGRATION
Although the monarch butterfly isn’t only located in North America, there is no evidence nowadays showing that the other populations of monarch butterflies do such a long migration. It’s believed that the fact that only these populations of butterflies go on a trip this long is due to the wide spreading of plants of the Asclepias genus over all North America that took place in the past. Scientists suggest this event allowed the monarch butterflies to spread progressively to the south.
WHICH PLACES DO THE BUTTERFLIES VISIT?
A migration is always a complex process. In this case, the migration to the south is divided into two simultaneous migrations:
- The east migration: this trip is made by those butterflies that fly from the east of the Rocky Mountains, South of Canada and a big part of USA to the central part of Mexico (90% of all the monarch butterflies located in North America go on this trip).
- The west migration: this trip is made by those butterflies that fly from the west of the Rocky Mountains, South of Canada and a little part of USA to the California coast (10% of all the monarch butterflies located in North America go on this trip).
Once in the winter habitats, the butterflies plunge into a lethargic state until the next spring, when they become sexually active and start mating before heading again to the north.
It’s a very surprising event seeing all the butterflies sleeping together and covering all the plants and trees of the winter habitats!
There exists a lot of protected areas all over the places where the butterflies go through.
One of the most important protected areas is the Monarch Butterfly Biosphere Reserve (Mexico City), which is considered a World Heritage Site by the UNESCO since 2008.
And why are these butterflies so protected? Besides the fact that their migration pattern is considered an incredible phenomenon, they are pollinators that contribute to the pollination of the wild flora and also of the crops of North America.
THE ‘QUEEN’ IS IN DANGER!
Although there exists a huge effort to protect them, the migratory phenomenon of the North American monarch butterflies is in danger nowadays due to the anthropic pressure, which could also put their populations at risk in the future.
According to the data generated by the WWF, the surface of the winter habitats occupied by these butterflies has decreased 94% in 10 years, going from 27,48 acres occupied in 2003 to only 1,65 acres in 2013. This is the lower value registered in the last 20 years.
Even though the surface occupied by these organisms has been fluctuating over the years as a part of a natural process, this pronounced decreasing that has taken place in only a few years suggests that butterflies are stopping their annual migrations to the south.
This recession has also been registered in other species of butterflies at different emplacements all over the world, so there must exist some kind of factor in common with the ones affecting the North American monarch butterfly populations.
WHAT COULD BE THE MAIN CAUSES OF THIS RECESSION?
According to the WWF, the main causes that could being putting in danger the migration process of the monarch butterflies are:
- The reduction of the surface occupied by plants of the Asclepias genus: as we said above, the caterpillars feed exclusively on these plants. But the use of certain herbicides and the changes on the rain patterns could being limiting their dispersal over a big part of North America.
- Deforestation: cutting down trees massively and the subsequent desertization could being reducing their winter habitats.
- Extreme climate: the global change, which entails changes in temperature and rain patterns, could being putting at risk the survival of adult butterflies, preventing them to reach the longevity required to carry out complete migrations.
WHICH EFFORTS ARE MADE TO PROTECT THESE POPULATIONS?
As I said above, monarch butterflies are an essential part of the pollination net of North America and also iconic insects, so there exists a big interest on protecting them.
Nowadays, most of the protected areas of North America are making a big effort to improve the quality of their habitats. Among them, the Monarch Butterfly Biosphere Reserve (Mexico) along with the WWF are trying to restore the woods where butterflies hibernate and also promoting a sustainable tourism (enter this link to see more information).
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The case of the monarch butterfly is only one of a huge list of animals in danger. Nowadays, a lot of animals with complex migration patterns and wide spreading areas are suffering similar pressures, mostly of them with an anthropogenic origin. There’s still so much work to do, and it depends on all of us.
- WWF – Monarch Butterfly
- Monarch Butterfly information
- Monarch Butterfly Fundation
- Scientific American – “Monarch butterflies Could Gain Endangered Species Protection”
- CBC News – “Why the monarch butterfly migration may be endangered”
Main picture by Carlos Adampol Galindo on Flickr.